North Koreans gather at a rally at Kim Il Sung Square in downtown Pyongyang, North Korea, on Friday. Tens of thousands of North Koreans turned out for the mass rally at the main square in Pyongyang in support of their leader Kim Jong Un’s call to arms.

SEOUL, South Korea — North Korean state media said Friday that the country’s leader, Kim Jong Un, had ordered his missile units to be ready to strike the United States and South Korea, which South Korean officials said could signal either preparations for missile tests or just more blustering.

The U.S. criticized the North Korean threat, which came one day after U.S. forces had carried out an unusual practice bombing exercise with advanced aircraft across South Korea.

“The United States is fully capable of defending itself and our allies,” said Lt. Col. Catherine Wilkinson, a Pentagon spokeswoman in Washington. “North Korea’s bellicose rhetoric and threats follow a pattern designed to raise tensions and intimidate others.”

The back-and-forth was viewed with worry by China and Russia. China’s Foreign Ministry reiterated its calls for restraint. Russia was more explicit, with its foreign minister, Sergey V. Lavrov, telling reporters in Moscow that he was increasingly concerned about a situation that could “get out of control — it will descend into the spiral of a vicious cycle.”

Kim’s order, which North Korea said was given during an emergency meeting early Friday, was similar to the one issued Tuesday when the North’s top military command told all its missile and artillery units to be on the “highest alert” and ready to strike the United States and South Korea in retaliation against their joint military exercises.

But by attributing such an order to its top leader, North Korea tried to add weight to its threat.

“We believe they are taking follow-up steps,” said Kim Min-seok, spokesman of the South Korean Defense Ministry, referring to increased activities of the North Korean military units. “South Korean and U.S. intelligence authorities are closely watching whether North Korea is preparing its short-, medium- and long-range missiles, including its Scud, Rodong and Musudan.”

He did not elaborate. But government officials and South Korean media said that there had been a surge in vehicle and troop movements at North Korean missile units in recent days as the United States and South Korea has been conducting joint military drills. The national news agency Yonhap quoted an anonymous military source as saying that North Korean vehicles had been moving to Tongchang-ri near the North’s western border with China, where its Unha-3 rocket blasted off in December.

North Korea might be preparing for an engine test ahead of a long-range rocket test, the source was quoted as saying. Scud and Rodong are the North’s mainstay short- and medium-range missiles. The Musudan, deployed around 2007 and displayed for the first time during a military parade in the North Korean capital, Pyongyang, in 2010, is a road-mobile intermediate-range ballistic missile with a range of more than 1,900 miles, according to the South Korean Defense Ministry.

In an angry reaction to the sanctions that the U.N. imposed after North Korea’s launching of a three-stage rocket in December and its third nuclear test last month, the North has repeatedly threatened to strike Washington, as well as the U.S. military bases around the Pacific and in South Korea, with nuclear-armed long-range missiles.

A photo released by the North’s official Korean Central News Agency on Friday showed Kim Jong Un conferring with his top generals on what the agency called “plans to strike the mainland U.S.” A military chart behind them showed what appeared to be trajectories of North Korean missiles hitting major cities in the United States.

North Korea also said its leader, Kim Jong Un, “finally signed the plan on technical preparations of strategic rockets of the KPA, ordering them to be on standby for fire so that they may strike any time the U.S. mainland, its military bases in the operational theaters in the Pacific, including Hawaii and Guam, and those in South Korea.” KPA stands for the Korean People’s Army.

Kim Min-seok, the South Korean spokesman, said the North’s “unusual” public announcement of such plans was partly “psychological.” Many experts and South Korean officials doubted that North Korea has such long-range missiles, much less the know-how to make a nuclear warhead small enough to mount on such rockets.

But other analysts believed that the North’s new KN-08 missiles, which were put on public display last April, were indeed intercontinental ballistic missiles, although they and Musudan have never been test-launched before. They wondered whether North Korea might use the current tensions as an excuse to launch them.

The country is barred from launching ballistic missiles under U.N. sanctions. North Korea’s development of the KN-08 was one of the reasons the Pentagon cited last Friday when it announced a $1 billion plan to add more missile interceptors in Alaska to better protect the United States against a potential North Korean missile attack.